1. Field of the Invention
This invention relates to an apparatus and method for measuring two-phase flow (liquid/gas) or three-phase flow (liquid/liquid/gas) of fluids.
2. Discussion of Background
The measurement of oil, water and gas flow finds application in various fields. In oil production, it is required for reservoir control and fiscal reasons. High accuracy of measurement is necessary as well as small instrumentation space requirements. Additional applications exist in petrochemical, nuclear and other industries.
In the past, three principal methods have been utilized for flow measurements.
As disclosed in U.S. Pat. No. 4,760,742, the gas in a liquid is physically separated from-the liquid, and each fluid is measured separately. A water-cut monitor is used to measure the amount of the water and the oil in the liquid phase. Two conventional single-phase flow meters are used to measure the gas and the liquid flow rates. This method can yield high accuracy, but requires gas-separating devices which are either very large or are very sensitive to flow rates and the liquid's viscosity, surface tension, etc.
A second approach is described in U.S. Pat. Nos. 4,168,624 and 4,050,896, wherein the total flow is measured at two different flow conditions (for example: different temperatures and different pressures along the pipeline). The changing of the gas volume during the change of this condition makes it possible to calculate the flow rates of the gas and the liquid. To achieve high accuracy in this method, a large difference in flow conditions between the two flow meters is required. This requires a large pressure drop, which is costly in terms of pumping energy.
A third technique as described by Baker, "Measuring Multi-Phase Flow", Chemical Engineer, No. 453, pp. 39-45, October, 1988, and Reimann et al, "Measurement of Two-Phase Mass Flow Rate: A Comparison of Different Techniques", Int. J. of Multi-Phase Flows, Vol. 8, No. 1, pp. 33-46, 1982, measures the total momentum flux, total density, total volumetric flow rate, and the water cut. All are required to calculate the amount of gas, oil and water. One such device uses the combination of a turbine flow meter, a venturi flow meter, a gamma ray densitometer or void fraction meter and a water-cut monitor. The advantage of this method is that it enables the use of venturies which have low pressure drops. The weak link in this technique is the densitometer, which is sensitive to the flow characteristics and the fluid's contaminants (heavy metals, etc.).
In many multi-phase flow applications it is desirable to predict the pressure drops which will occur in various piping apparata with different combinations of multi-phase fluids. This information is critical to piping design, pump sizing, etc. While information has been compiled on the pressure drops of a two-phase fluid comprising of water and air, it has not been possible to predict the pressure drops for other, more unique multi-phase fluids.